Mobile communications systems are made up of a plurality of cells or cell sectors. Each cell or cell sector provides a radio communications center through which a mobile station establishes a call or other communications session with another mobile station or a terminal connected to either a circuit-switched network (e.g., public-switched telephone network or PSTN) or a packet-switched data network. Each cell or cell sector includes a base station (or access point) and a base station controller (or radio network controller) to enable communications with mobile stations in the cell or cell sector.
Wireless networks are capable of carrying both circuit-switched and packet-switched traffic (e.g., voice traffic, data traffic, etc.). Examples of wireless networks that support communication of packet-switched traffic include those that operate according to the GSM (Global System for Mobile) and UMTS (Universal Mobile Telecommunications System) standards, as defined by the Third Generation Partnership Project (3GPP).
Other types of wireless networks include enterprise and residential wireless local area networks (WLANs). A WLAN refers to a local area network that mobile stations can access wirelessly. A WLAN is a private network, either owned by an organization or municipality (enterprise) or by an individual. A WLAN is usually secured such that only authorized users are allowed to use the WLAN. A WLAN differs from a public cellular wireless network in that the WLAN is limited for use by users of a specific enterprise or a group, whereas the public cellular wireless network is for general use of subscribers of the cellular wireless network. Examples of standards that define WLANs include IEEE (Institute of Electrical and Electronic Engineers) 802.11, 802.11a, 802.11b, 802.11g, Bluetooth, WiMAX (Worldwide Interoperability for Microwave Access), 802.16, 802.20, and so forth. Technologies employed in such networks can include spread spectrum technologies such as CDMA and orthogonal frequency division multiplexing (OFDM).
When a mobile station establishes a data session with an external network device that is located outside the wireless network, the external network device perceives the mobile station as being located at an access gateway that couples wireless network devices to an external data network (e.g., Internet or other type of data network). In the WLAN context, this access gateway is referred to as a packet data gateway (PDG). For a GSM or UMTS wireless network, the access gateway is referred to as a GGSN (Gateway GPRS Support Node). The access gateway is the point to which data packets containing the mobile station's external IP (Internet Protocol) address are routed.
Typically, several GGSNs and PDGs are deployed in a wireless network, with the multiple GGSNs and PDGs performing load sharing to enhance bandwidth for packet data sessions. The GGSNs and PDGs can be considered functions deployed on corresponding access gateway nodes. A mobile station is able to transition (hand off) between a GSM/UMTS access network and a WLAN access network. If the mobile station is initially connected to a first wireless access network (e.g., a GSM/UMTS access network) and has an existing data session with a particular access gateway node, and the mobile station transitions to a second wireless access network (e.g., a WLAN access network), there is no way to guarantee that the new packet data session using the second wireless access network will be hosted by the same access gateway node as that used with the first wireless access network. If the packet data session switches to a different access gateway node as a result of the transition (handoff), then the external IP address for the mobile station may change. Changing the external IP address of the mobile station during a data session is associated with various undesirable issues.